I think the amount of 1st/3rd parties depends on things we just can't really say right now.
For example, just because Spirits don't disconfirm fighters, it doesn't mean that we're getting a pass full of them. I'd actually say I don't personally think we're getting a full upgrade pass.
If Assist Trophies are out (which I think there's a more compelling case for, whereas Spirits are essentially the trophy equivalent), then that takes out Waluigi, Isaac, Lyn, and others. If post-launch Spirit event timing and purpose disconfirm, that would knock out Pokemon (though, it's Pokemon, so I'm not quite willing to say it's out), Astral Chain, and Ring Fit Adventure. That basically leaves Rex from XC2 and Bandanna Waddle Dee as front runners for 1st party DLC.
If other trends for Season Pass DLC continue, such as picking a/the protagonist of each game/series then that looks good for Rex, not so much for BWD. That significantly cuts down on popular 1st party picks even further. While this is a “trend”, I don’t think it’s as strong as the overall series trend of “mascot or protagonist first for new series or games”. Has been consistent but could change.
There's also the flipside of things, where they add an Assist Trophy as playable or another Spirit Upgrade aside from Rex, for example. This is holistically different from the outlook for 3rd parties, which still has extensive options to go for.
I would just be wary of how we look at certain things with character ratios. Obviously everyone has different wants when it comes to Smash, especially if this is the last wave of DLC. I think, personally, the safest bet is 3:3, split down the middle between 1st and 3rd parties. I could however see it swing to 4:2 with 3rd parties receiving more just given the situation, and uncertainties obstructing the paths of certain 1st party additions.
This wasn’t meant as a “1st parties bad” post. Simply how I view the situation currently.
EDIT: One last thing, we DO have a bit of information of how post-launch spirits COULD be treated. Three Houses wasn't celebrated with Spirits because it would later be added as an entire Spirit Board with a DLC fighter. This goes directly against Pokemon Sword & Shield, Ring Fit, or Astral Chain.
Then there's the opposite, where ARMS had base game Spirits that were simply incorporated into a DLC Spirit board. I think it's a toss up, but that's largely why I'm unsure of how it'll work. Seems to me that Pokemon relies of post-launch Spirits not being a disconfirmation.